Physical vapor deposition by sputtering is a well known process that has found widespread application in the fabrication of very large scale integrated (VLSI) semiconductor devices. In magnetron sputtering a plasma is formed in a low pressure inert gas. The plasma is confined to a region near a sputter target, which is made of the material to be sputtered and which usually serves as the cathode of the system. A magnetic field, typically having field lines which loop through the sputter target surface, restricts the trajectories of the electrons in the plasma, thereby intensifying and confining the plasma. Ions in the plasma bombard the sputter target dislodging atoms of the target material which are then deposited on a substrate.
A large number of VLSI devices are typically fabricated on a thin, generally circular silicon substrate referred to as a wafer. VLSI device fabrication involves a large number of processing steps, with sputtering being used to provide metallization layers and interconnects between device layers. Most commonly, sputtered aluminum is the material used for these purposes.
In recent years wafer sizes have continually increased, and now the use of eight-inch diameter wafers is common in the industry. Large wafer sizes permit a larger number devices to be grown on a single substrate. However, larger wafer sizes impose greater demands on sputtering systems. For example, one requirement of a sputtering system used in semiconductor processing is that it deposit a layer of uniform thickness over the entire wafer surface. (Hereinafter the term uniformity will be used in connection with the thickness of the deposited film unless the context suggests otherwise.) Lack of uniformity may result in lowered device yield (i.e., the percentage of devices which meet operating specifications) and/or variations in device performance. Larger wafer sizes make it more difficult to achieve very demanding levels of uniformity.
One approach to improving the uniformity of a sputtering system is to sputter from two concentric targets. For an example of this approach see U.S. Pat. No. 4,606,806 which describes a sputtering source sold by the assignee of the present invention under the trademark ConMag.sup..RTM. II. In the ConMag.sup..RTM. II sputtering source each of the sputter targets has a unique shape and its own separate power supply enabling separate control over the sputtering rate from each target. In addition to providing good uniformity, this configuration provides good step coverage, i.e., the ability to uniformly cover vertical "steps" and other angled surfaces on the wafer.
A number of commercially available sputtering sources use a planar sputtering target. One such source, commercially available from the assignee of the present invention under the trademark VersaMag.sup.#, relies on a rotating magnet mounted behind the target for moving the plasma over the face of the target. Rotation of the plasma was introduced for the purposes of improving uniformity and step coverage, as well as improving the uniformity of target erosion so that targets are more efficiently utilized.
In the recently issued U.S. Pat. No. 4,995,958, entitled Sputtering Apparatus With A Rotating Magnet Array Having A geometry For Specified Target Erosion Profile, which is also assigned to the assignee of the present invention, it was shown how to construct a closed-loop rotating magnet, for use with a planar sputtering source, which enables one to realize a predetermined erosion profile to thereby achieve, for example, highly efficient target material utilization and high deposition rates. The invention of the '958 patent is readily adapted to use in a VersaMag.sup.# sputtering source.
A closed-loop magnet configuration of the type described in the '958 patent has the additional advantage of being easily adjustable so that the shape of the magnet array, and therefore the characteristics of the sputtering source, can be changed without great difficulty or expense. As described in that patent, the disclosure of which is incorporated by reference, a plurality of magnets are held in position by two iron keepers which define the shape of the closed loop. Replacement and/or adjustment of the iron keepers to provide a different closed-loop configuration is a relatively simple matter. In this manner it is possible to use one source for different purposes.
As noted, the original objective of the closed-loop rotating magnet of the '958 patent was to achieve better target utilization efficiency, normally an important objective given the high cost of sputter targets, and to achieve high deposition rates, another important factor due to the demand for ever greater system "throughput". In some instances, the need for greater uniformity might outweigh the desire for efficient target utilization and deposition rate. In such a case the high levels of uniformity achieved by multiple sputtering rings remain attractive.
In this context the ability to provide a multiple ring sputtering source which is adaptable to other uses, and without the need to design an entirely new source is especially desirable.
Accordingly, it is an object of the present invention to provide a multiple ring sputtering source capable of highly uniform deposition.
Another object of the present invention is to adapt existing rotating magnet technology to provide a multiple ring sputtering source.